In a Dash 8 300 or 200 it takes about 8 nautical miles to slow from 240 knots to 150 knots while descending at around 1000 - 1500 fpm clean, once the gear, flap, and condition levers are down/forward it takes very little time to come further back to approach speed (about 120 knots).
If you slow down while flying level it'll happen a lot quicker. If you put the condition levers forward first (not SOP for us) you can slow it down very quickly (or achieve a high rate of descent while slowing down.)
Last edited by AerocatS2A; 5th Jun 2012 at 05:10.